This memo was prepared with assistance from Claude Opus 4.6 (Anthropic) for literature search, synthesis, and technical writing. All content was reviewed and verified by Al Irvine, R.P.Bio.

Figure 0.1: Study area showing the seed collection zone and proposed planting areas. Data: BC Freshwater Atlas, BC Geographical Names, BEC (BC Data Catalogue).

1 Question

Can black cottonwood (Populus trichocarpa) plugs collected along the Nechako River — between Prince George and Kenny Dam (~54°N) — be used for riparian restoration at two sites further north?

  1. Mackenzie basin — 75–150 km north of Prince George (~55–56°N)
  2. Skeena / Neexdzii Kwah — 120–150 km north of Kenny Dam (~54.5–55°N)

Both transfers move material 1–2° latitude northward, within the Sub-Boreal Spruce (SBS) biogeoclimatic zone, and within BC’s interior where gene flow among cottonwood populations is relatively continuous.

2 What the Literature Says

2.1 Genetic structure: interior vs. coast

The most important finding for this question is that interior P. trichocarpa populations are genetically well-connected, unlike coastal populations where sharp ecotypic breaks exist across short distances. Xie et al. (2012) documented strong ecotypic differentiation among coastal populations in northern BC (Nass, Skeena, and Kitimat drainages), but this pattern reflects isolation in distinct coastal valleys separated by mountain barriers. Interior populations connected by broad river systems — like the Nechako, Fraser, and upper Skeena — show much higher gene flow and more gradual (clinal) trait variation.

Genome resequencing of range-wide samples found no evidence of restricted gene flow among interior BC drainages, with population structure following smooth isolation-by-distance gradients rather than drainage-specific breaks (Slavov et al. 2012). A landscape genomics analysis of 498 accessions across 25 drainages confirmed that interior populations are highly admixed with low differentiation — the sharp genetic boundaries documented on the coast do not exist in the interior plateau (Geraldes et al. 2014).

2.2 Clinal variation in adaptive traits

Cottonwood shows strong clinal variation in traits that matter for survival at new latitudes. McKown et al. (2014) found that bud set timing, bud flush timing, growth rate, and leaf morphology all vary predictably with latitude across the species’ range. These trait shifts are largely driven by photoperiod — specifically the FLOWERING LOCUS T genes (FT1 and FT2) — rather than temperature alone.

Evans et al. (2019) confirmed that local adaptation in P. trichocarpa follows both latitudinal and elevational gradients, with genomic signatures of selection concentrated in genes controlling phenology and cold hardiness.

2.3 Inter-basin gene flow: wind, pollen, and drainage divides

A key consideration for this transfer is that the source (Fraser/Nechako) and destinations (Mackenzie, Skeena) lie in different major river basins. However, multiple lines of evidence indicate that drainage divides in the interior are not barriers to gene flow in cottonwood.

Cottonwood seeds — equipped with cottony trichomes for wind dispersal — can travel up to 30 km under storm conditions (Nathan et al. 2002). The drainage divides between the Nechako, upper Skeena, and Parsnip/Pack (Mackenzie system) cross low-elevation passes on the interior plateau (~800–1000 m), not alpine barriers, making cross-basin wind dispersal physically plausible. A global analysis of 1,900+ tree populations confirmed that wind-connected populations are more genetically similar, with prevailing wind patterns shaping asymmetric gene flow and diversity gradients (Kling and Ackerly 2021).

Pollen dispersal reinforces this connectivity. Direct paternity analysis in P. trichocarpa found pollen immigration rates of 32–54% from outside local populations, demonstrating that a large fraction of successful pollination originates beyond the immediate stand (SLAVOV et al. 2009). Hydrochory (water-mediated dispersal) moves seeds downstream within drainages but cannot cross between basins — inter-basin connectivity depends on wind transport of seeds and pollen.

The available genomic evidence suggests that genetic structure in interior cottonwood follows smooth latitudinal gradients rather than drainage-specific breaks. Moving material between the Fraser, Mackenzie, and Skeena basins in the interior plateau appears consistent with natural gene flow patterns.

2.4 What the clines mean for 1–2° transfers

The clinal patterns are gradual within the interior. Moving material 1–2° north means the plugs will encounter:

  • Slightly longer winter nights — photoperiod at 55–56°N triggers bud set a few days earlier than at 54°N, so Nechako-source trees may set buds slightly later than local genotypes. In practice, this means a small increase in early-frost risk in the first few autumns.
  • Comparable growing-season temperatures — within the SBS zone, mean growing-season temperatures are similar across this latitudinal band, especially given that climate warming is shifting thermal regimes northward.

The key question is whether the phenological mismatch is large enough to cause mortality. For a 1–2° shift within the interior, the literature suggests the risk is low — certainly lower than the large mortality events documented for coastal ecotype transfers (Xie et al. 2012).

3 BC Regulatory Context

3.1 Climate-Based Seed Transfer (CBST)

BC’s seed transfer system has moved from fixed geographic Seed Planning Zones to a climate-based approach that matches seedlots to planting sites by climate variables (O’Neill et al. 2008; Pelai et al. 2021). Under CBST, the question is whether the climate envelope at the collection site overlaps with the climate envelope at the planting site — and for Nechako-to-Mackenzie or Nechako-to-Skeena, there is substantial overlap within the SBS zone.

However, CBST was designed primarily for reforestation obligations under the Chief Forester’s Standards for Seed Use. It is less clear whether CBST formally applies to restoration plantings outside of timber tenures. In practice, restoration practitioners in BC routinely move riparian material across similar distances without formal seed transfer approvals, but this is a gap in the regulatory framework (Pelai et al. 2021).

3.2 Seed sourcing for restoration under climate change

The broader restoration literature supports “climate-adjusted provenancing” — intentionally sourcing material from slightly warmer or lower-latitude sites to pre-adapt plantings to anticipated climate conditions (Kramer et al. 2015; Pedrini et al. 2022). Under this logic, Nechako-source material may actually be better suited to future conditions at the northern sites than strictly local genotypes, because the Nechako’s current climate approximates the projected future climate at 55–56°N.

4 Recommendations

  1. The transfer is likely appropriate. Moving Nechako cottonwood plugs 1–2° north within the interior SBS zone is well within the range of natural gene flow and clinal variation. The genetic risk is low compared to cross-ecotype transfers (e.g., coastal to interior).

  2. Mix provenances where possible. If local cuttings from the Mackenzie or Skeena sites are available, mixing them with Nechako-source plugs provides a genetic hedge. This composite provenancing approach is increasingly recommended for restoration under climate uncertainty (Kramer et al. 2015; Pedrini et al. 2022).

  3. Monitor fall phenology in the first 2–3 years. The main risk is a slight delay in bud set relative to local genotypes. Watch for frost damage in September–October, particularly at the Mackenzie sites which are the furthest north.

  4. Document the provenance. Record the exact collection location along the Nechako — closer to Prince George (~575 m elevation, SBSwk1) vs. closer to Kenny Dam (~850 m, SBSdk) matters because the elevation and BEC variant affect the magnitude of any climate mismatch at the receiving sites. This information is essential for interpreting monitoring results and informing future transfers.

References

Evans, Luke M., Gancho T. Slavov, Athena D. McKown, Sheel Bansal, Robert D. Guy, and Thomas R. Thomas. 2019. “Phenotypic and Genomic Local Adaptation Across Latitude and Altitude in Populus Trichocarpa.” Genome Biology and Evolution 11 (8): 2256–72. https://doi.org/10.1093/gbe/evz151.
Geraldes, Armando, Nima Farzaneh, Christopher J. Grassa, et al. 2014. “Landscape Genomics of Populus Trichocarpa: The Role of Hybridization, Limited Gene Flow, and Natural Selection in Shaping Patterns of Population Structure.” Evolution 68 (11): 3260–80. https://doi.org/10.1111/evo.12497.
Kling, Matthew M., and David D. Ackerly. 2021. “Global Wind Patterns Shape Genetic Differentiation, Asymmetric Gene Flow, and Genetic Diversity in Trees.” Proceedings of the National Academy of Sciences 118 (17). https://doi.org/10.1073/pnas.2017317118.
Kramer, Andrea T., Jeremie B. Fant, Mary V. Ashley, and Pati Vitt. 2015. “Seed Sourcing for Restoration in an Era of Climate Change.” Natural Areas Journal 35 (1): 122–30. https://doi.org/10.3375/043.035.0116.
McKown, Athena D., Robert D. Guy, Jaroslav Klápště, et al. 2014. “Geographical and Environmental Gradients Shape Phenotypic Trait Variation and Genetic Structure in Populus Trichocarpa.” New Phytologist 201 (4): 1263–76. https://doi.org/10.1111/nph.12601.
Nathan, Ran, Gabriel G. Katul, Henry S. Horn, et al. 2002. “Mechanisms of Long-Distance Dispersal of Seeds by Wind.” Nature 418 (6896): 409–13. https://doi.org/10.1038/nature00844.
O’Neill, Greg, Alan Hamann, and Tongli Wang. 2008. Assisted Migration to Address Climate Change in British Columbia: Recommendations for Interim Seed Transfer Standards. No. 048. BC Ministry of Forests and Range. https://www.for.gov.bc.ca/hfd/pubs/Docs/Tr/Tr048.htm.
Pedrini, Simone, Adam T. Cross, and Kingsley W. Dixon. 2022. “Seed Sourcing Strategies for Ecological Restoration Under Climate Change: A Review of the Current Literature.” Frontiers in Conservation Science 3. https://doi.org/10.3389/fcosc.2022.938110.
Pelai, Ricardo, Shannon M. Hagerman, and Robert Kozak. 2021. “Seeds of Change? Seed Transfer Governance in British Columbia: Insights from History.” Canadian Journal of Forest Research 51 (7): 958–68. https://doi.org/10.1139/cjfr-2020-0235.
SLAVOV, G. T., S. LEONARDI, J. BURCZYK, W. T. ADAMS, S. H. STRAUSS, and S. P. DIFAZIO. 2009. “Extensive Pollen Flow in Two Ecologically Contrasting Populations of Populus Trichocarpa.” Molecular Ecology 18 (2): 357–73. https://doi.org/10.1111/j.1365-294X.2008.04016.x.
Slavov, Gancho T., Stephen P. DiFazio, Joel Martin, et al. 2012. “Genome Resequencing Reveals Multiscale Geographic Structure and Extensive Linkage Disequilibrium in the Forest TreePopulus Trichocarpa.” New Phytologist 196 (3): 713–25. https://doi.org/10.1111/j.1469-8137.2012.04258.x.
Xie, Chang-Yi, Michael Carlson, and Cheng C. Ying. 2012. “Ecotypic Mode of Regional Differentiation of Black Cottonwood (Populus Trichocarpa) Due to Restricted Gene Migration: Further Evidence from a Field Test on the Northern Coast of British Columbia.” Canadian Journal of Forest Research 42 (2): 400–407. https://doi.org/10.1139/x11-187.